In programmable logic integrated circuit devices, antifuses may be used to programmably connect conductive lines to each other. An antifuse is a structure which is non-conductive when manufactured but which can be caused to become permanently conductive by application of sufficient voltage across its terminals.
The art of forming antifuses which may be reliably used in integrated circuit devices has evolved over the last ten years, with a variety of antifuse structures becoming commercially available.
FIG. 1 shows one prior art antifuse structure manufactured using a known process. A lower conductive terminal 11 is formed of polycrystalline silicon. As shown in the view of FIG. 1, poly layer 11 may be thought of as extending perpendicular to the plane of the drawing. Formed on the poly line is an insulation layer 13, which is patterned to form an opening 13a for antifuse contact. An amorphous silicon layer 14 about 1500 .ANG. thick is formed on this insulation layer, using silane gas (SiH.sub.4) in a low pressure chemical vapor deposition process at a temperature of about 560.degree. C. This amorphous layer 14 is patterned to form antifuse 14a. Formed above antifuse 14a are layers of titanium metal 15, then titanium nitride 16, then aluminum or an aluminum-silicon alloy 17. These three layers 15, 16, and 17 are patterned simultaneously to form a conductive line which makes contact to the upper terminal of antifuse 14a.
FIG. 2 shows another prior art antifuse structure. The lower conductive terminal is formed from aluminum or aluminum-silicon alloy 21 on which is formed titanium-tungsten layer 22. After a dielectric layer 23 (silicon dioxide for example) is formed and patterned to expose a titanium-tungsten contact area 23a, amorphous silicon antifuse 24a is formed by depositing amorphous silicon 24 using a PECVD process, and patterning the amorphous silicon 24 to leave the antifuse 24a extending into the contact 23a and contacting the lower conductive layer 22.
PECVD amorphous silicon has relatively low conductivity. However, the conductivity is sufficient that either the antifuse will have an undesirably high leakage in its unprogrammed state or the antifuse must be thick enough to require high programming voltage. To reduce leakage to an acceptable level, the amorphous silicon layer 24 is made to be about 1500 .ANG. thick.
Upper conductive layers of titanium 25, titanium-tungsten 26, and aluminum or aluminum-silicon alloy 27 are formed above antifuse 24a.
Prior art antifuse structures are discussed in European Patent Application EP 0 416 903 A2 of QuickLogic entitled "Method of Fabricating a Programmable Interconnect Structure" and in McCollum et al., U.S. Pat. No. 5,070,384 assigned to Actel Corporation. The Actel structure includes a dielectric layer between the lower refractory metal and the amorphous silicon layer and is shown in FIG. 5. FIG. 5 is taken directly from U.S. Pat. No. 5,070,384 and the reference numerals have not been changed. In FIG. 5, layer 20 is titanium, and layer 24 is a thin dielectric layer which may be formed by oxidizing the surface of titanium layer 20 and may be approximately 100 .ANG. thick. Layer 26 is amorphous silicon on the order of 2000 .ANG. thick and may be doped with phosphorus or arsenic to a level of about 10.sup.20 atoms/cm.sup.3 to lower resistance. Upper electrode 28 may be of tungsten, molybdenum, platinum, titanium, titanium nitride, tantalum, silicides of those metals, or arsenic doped polysilicon.
The past antifuses have required a fairly high programming voltage, on the order of 12-14 volts. Indeed the Actel patent describes programming at 20 volts (see col. 9 line 1). Standard transistors used in 5-volt systems typically break down at about 12-14 volts, which means that special processing is needed to enhance the breakdown characteristic of the transistors for programming the antifuses, and care has to be taken for isolating other structures from antifuse programming voltages. Also, in a submicron process with a relatively thick layer of amorphous silicon, step coverage of the top electrode extending into the antifuse via opening is undesirably poor.